Coating stainless steels

ABSTRACT

Films or coatings on stainless steel produced by immersion in solutions of chromic acid are electrolytically treated to increase the coating&#39;&#39;s resistance to abrasion. In the electrolytic treatment, the coated stainless steel is made the cathode in a bath of an aqueous solution of chromic acid.

United States Patent 11 1 1451 Au .'2s, 1973 Hart [ COATING STAINLESS sTEELs' 2,219,554 10/1940 Batche11er 148/614 48 [75] Inventor: Anthony Christopher Hart, Dudley, 5/1942 Balchener 1 I31 England FOREIGN PATENTS OR APPLICATIONS 73 Assigneez The International Nickel p y, 448,565 5/1948 Canada.. 204/38 R Inc. New York NY 1,187,785 4/1970 Great Br ta n 1,097,197 12/1967 Great Bntam 204/140 [22] F11ed: Feb. 10, 1971 OTHER PUBLICATIONS [21] Appl' 4 Theory of Corrosion and Protection of Metals by N. D.

Tomashov, The MacMillan Co. (1966), page 327. [30] Foreign Application P i it D t The Corrosion Handbook edited by H. H. Uhlig, John May 26, 1970 Great Britain 25,215/70 'l 1 P 22 and Jan. 11, 1971 Great Britain 1.245/71 5131111885 Steel Fabncatlo" by Allegheny Ludlum Steel 7 Corp., 1959 pp. 346 and 347. [52] US. Cl 204/140, l48/31.5, 204/35 N,

204/38 R, 204/56 R Primary Examiner-John H. Mack [51] Int. Cl. C23b 5/50, C23f 17/00, C23b 9/00 Assistant Examiner-T Tufariello [58] Field of Search 204/140, 35 N, 29, yM r L. nel

204/35 R, 38, 56 R, 18 R; 148/315 [57] ABSTRACT [56] References Cited Films or coatings on stainless steel produced by immer- UNITED STATES PATENTS sion in solutions of chromic acid are electrolytically 1,827,204 10/1931 Mason 204/140 treated to increase the coatings resistance to abrasion. 1.327.247 /1931 Mason 204/140 In the electrolytic treatment, the coated stainless steel 1945151 2/1934 Edwards r r 204/35 R is made the cathode in a bath of an aqueous solution of 3,081,238 3/1963 Gurry 204/56 R chromic acid 3,210,220 10/1965 Clegg et a1 148/621 2,172,353 9/1939 Batcheller 148/624 10 Claims, N0 Drawings The natural finish of stainless steelas it comes from the mill is quite attractive and, in most cases, that is the state in which this material has been utilized. For-some purposes, architectural applications, for example, col ored stainless steels would be desirable to achieveadditional variety in the visual impact of this material. A"

number of proposals exist, therefore, for coloring stainless steel, among them the processes described in British Pat. Nos. 1,122,172 and 1,122,173, both published July 31, 1968. In accordancewith these British patents, the stainless steel is immersed inan aqueous solution of chromic acid which also contains sulphuric acid and," in the latter of these patents, also containing manganous sulfate. Thus, the British patents disclose that the coloring bath may contain CrO in an amount between 50 and 1,000 g/l. (grams per liter) of water, and canbe further acidified with sulfuric acid in amounts up to 600 g/l. A preferred ratio of the weights of Cr(), to sul-' fate ion added as sulfuric acid is from' about 100:1 to about 1:1, although in'some cases the ratiocan extend to 1:2. An addition of manganous sulfate (MnSO -4- H O) in an amount in the'range of 0.3 g/l. to 50 g/L, e.g., 4 to 5 g/l., speeds the formation of the colored films. The bath temperature should be in the range from about room temperature up to about 100C., e.g.,

70C. Blue, yellow, red, greenish-red and gold films have been produced on stainless steels by this immersion process. In addition, it is disclosed that a so-called terminal gold color can be. produced by immersion in the above-described solutions, e.g., a chromic acidsulfuric acid bath at a temperature of 65 to 75C. containing 520 g/l. to 580 g/l. of CrO, and a Cro -sulfate ion ratio of from 3:1 to 7 :1, which color is not changed by increasing the period of immersion. It is possible, however, to obtain. blue to red colored films from such terminal gold film by heat treating in air at a temperature of from 300 to 7000C.,

By the processes just described, attractive colors are a period of time which depends upon the concentration 7 of the bath, the current density and the temperature, but which is alwaysat least' 1 minute, though notso long that any chromium becomes visible on the colored surface as a white deposit. lt is'desirable to make the current density low, because it is then more easily possible to stop the process at the right time by visual observation.

ltis found that b-y'm'eansof the invention stainless steel treated by the aqueous solution to form 'both' colproduced on the stainless steel which are as resistant to scratching and abrasion as the original steel surface, but like that surface, the coatings can be removed if subjected to rubbing with an abrasive member such as an eraser incorporating abrasive particles. An improvement in the resistance to abrasion of these colored coatings would certainly make these colored coatings. suitable for a wider range of applications.

In the production of colored films the steel must be left in the solution for a specific period of time beforeany recognisable color appears onthe surface. We have found, however, that some change occurs inv the surface of the steel before any color is detectable, the steel retaining its natural appearance.

The corrosion resistance of stainless steel, whether austenitic or ferritic, is well known but nevertheless susceptible of improvement. Our main object in the invention is to improve this resistance. Another object is to improve the resistance of colored stainless steel to both corrosion and abrasion.

Other objects and advantages of the invention will become apparent from the following description.

According to the invention a film is produced on the surface of stainless steel by treatment with an aqueous solution of chromic acid with or without other constitubred and uncolored (that is-to say, natural colored) steels are rendered more resistant to corrosion by the application of the'cathodic treatment and the resistance to abrasion of colored steels is muchimproved.

Coloringmay preferablybe achieved by'immersing the-stainless steel in a hot aqueous solution containing chromicand sulphuric acids, thecomposition and temperature range being as follows:

Range Optimum CrO, concentration 200-400 gll 300 g/l 11,80. concentration 350-700 g/l- 550 g/l (SS-80C C Temperature The coloring treatment can be successfully operated over wide limits of solution concentration and temperature, but in order to achieve reproducibility of results and good color matching it is necessary to ensure that these variables and also .thetime of immersion are closely controlled. Under the optimum conditions coloring takes between about 7 and 15 minutes, depending on theshadedesired. At lower temperatures the time required for coloring is lengthened, whilst at higher temperatures the time is shorter and consequently color control may be difficult. Under the optimum conditions a useful uncolored film is formed in about 5 minutes, a blue film in about 8 minutes and a gold film in about 10 minutes.

The cathodic treatment of the film-bearing stainless steel is most advantageously carried out in an electrolyte containing 250 g/l. CrO, and 2.5 g/l. sulfuric acid at a temperature of 40C. The most advantageous current density is from 2.4 to 4.8 amp/dm. The period of treatment will then normally be from 4 to 15 minutes, e.g., from 7 to 10 minutes. However, there may be considerable departure from these conditions and the concentration of CrO is at least 25 gll.,' preferably from 100 to 750 g/l., and that of sulfuric acid is broadly 0.1 to 100 g/l'., preferablyfrom l to 10 g/L, while the temperature may vary from 10 to about 100C., e.g., from 20 to C., and the current density may be in the range 0.3 to 30 ampldm., preferably 0.6 to 10 amp/dm., e.g., from about 1 to 5 amp/dm. lnaccor-' dance with the variations in these factors, the duration of the treatment may then be as short as one-half minute, e.g., 2 minutes, or as long as 30 minutes.

Care must be exercised during the treatment since ment as soon as possible after the coloring treatment in order to avoid the possibility of staining or marking of the unhardened film. Delay in application of the hardening treatment does not, however, prevent it being carried out effectively.

Both the coloring and hardening treatments can be carried out in lead-lined tanks heated by steam jackets, of the type used for conventional chromium plating. It is desirable to stir the coloring solution gently so as to maintain uniformity of temperature throughout it, but not to agitate either solution.

The resistance to corrosion can be tested by the CASS test described in BS 4601/1970, Appendedix G, and various test panels have been subjected to this test.

In the first set of tests (Test 1) panels of 18/8 chromium-nickel austenitic stainless steel polished to a mirror finish were immersed in the optimum coloring solution, some for about minutes to form uncolored films leaving the natural color of the steel, some for about 8 minutes to form blue films and some for about minutes to form gold films. Some of the panels thus treated were put aside for the CASS test, and others were sub- The improvement in-the abrasion resistance of colored films produced by means of the invention is shown by the resultsof tests in which panels bearing colored films are rubbed with a pencil eraser containing abrasive particles under a load of 500 g. and the number of rubs required to remove the coloring is noted.

Panels of an l 8/8 chromium-nickel stainless steel polished to a mirror finish were colored blue by immersion in an aqueous solution containing 300 g/l. of CrO and 550 g/l. of sulfuric acid at 70C for 8 minutes. The blue films thus formed were removed from some of these after from 2 to 4 rubs. The other panels were then given cathodic hardening treatment in aqueous solutions containing various amounts of chromic acid under various conditions of temperature, time of exposure and current density. The color of the test panels was virtually unaffected by this treatment. The colored and hardened panels were then rubbed, the tests being stopped after 200 rubs. The results are shown in Table 3 below in which 200+ means that the coating had not failed at this stage. I

TABLE 3 jected to the cathodic hardening treatment in an electrolyte containing 250 g/l. CrO and 2.5 g/l. sulfuric Cathodic Treatment Rubs to acid at a temperature of 40C. In another series of tests 50mm)" Composition p- Tilfflc fallurc C min. density (Test 2) slmilar panels were treated in the same way examps/(1m, cept that only films of natural and blue color were pro- 25 CrO +2.5 H SO 40 20 0.3 50 duced. The results of CASS tests on all these panels and u 40 20 (16 160 on untreated panels of the same steel in each case are 40 10 1.2 160 40 20 1.2 200 shown in Table l. u 40 10 2A 180 40 15 2.4 200+ TABLE I 40 20 2.4 200 4O 4 4.8 200+ 40 7 48 200+ Test Treatment Color Result 40 10 4.8 200+ 12 4.8 180 l Untreated Natural Moderately stained u i Uncolored Natural Moderately stained 40 [0 60 (olored Blue Heavily stained 40 6 130 (.olored (iold Heavily stained 40 2 '80 Uncolored and hardened Natural Very lightly stained .t 40 2 I20 Colored and hardened Blue Lightly stained 40 1 90 (.olored and hardened Gold Lightly stained 40 2 Untreated Natural Lightly stained 250g croTtz'sgtll H250 g8 Uncolored Natural Light/moderately u 60 20 100 flamed H 6O 10 4.8 40 Colored Blue Moderate/heavily 80 20 24 60 Stained so 10 4.8 Uncolored and hardened Natural Unstained 45 Colored and hardened Blue Unstained 25g/]Cr()*3+2 5g/| H330 0 20 120 40 7 4.8 140 40 10 4.8

it will be seen that the resistance to staining of the 00 IC 40 10 0 panels subjected to the hardening treatment was dis- 5O 1 g] rofilz'sglmzso 4O 20 105 tinctly better than that of those bearing unhardened 40 7 4.8 120 40 10 4.8 films.

In other tests panels of mirror finish 17% chromium 400g/l CrQ+2.5g]l H SO 40 to 2.4 I60 ferritic steel were similarly treated to produce un- :3 i-g {:8 colored and blue films and similarly tested with the re- 55 40 10 1 200+ sults shown in Table 2.

SOOg/l CrO,+2.Sg/l H,SO. 40 10 2.4 40 20 2.4 TABLE 2 40 7 4.8 I80 40 10 4.8 Treatment Color Result 60 7508: 003+ 25g" H2504 40 [O 2.4 180 V 40 20 2.4 Untreated Natural Moderate/light 40 7 4.8 120 staining 40 10 4.8 80 Uncolored Natural Moderate staining Colored Blue Moderate/heavy 250g CrO,1+ lOg/l H250 40 20 2.4 60

staining Uncolored and hardened Natural Moderate/light 65 40 m j; 71

staining Colored and hardened Blue Moderate/light 2502/ C r l g/H2 4 2 staining In further tests mirror finished panels of three ferritic stainless steels were treated, namely 17% chromium, 13% chromium, and 17% chromium-1% molybdenum, steels respectively. The panels were all treated to develop a blue color, and this was removed from some after less than 6 rubs in each case. The remaining panels were cathodically treated in an electrolyte containing 250 g/l CrO and 2.5 g/l. l'l SO at 40C for 20 minutes at a current density of 2.4 A/dm, and these maintained the color for from 80 to 160 rubs.

It will be seen that the resistance of the coating to abrasion was substantially improved by each of the cathodic treatments.

In carrying out the invention, mechanically abraded steel surfaces preferably are employed to obtain an even color.

Mechanical deformation does not appear to detrimentally affect the hardened coating or film, and the treated material can be bent, deep drawn and rigidized without cracking the film or reducing the color intensity of the deformed areas. In demonstrating this, the colored and hardened material has been bent through 90 without cracking or flaking the colored films orreducing the color intensity at the bent portion. Further, the material has been deep drawn to form a cup 35 mm. in diameter by 20 mm. deep without damaging the colored film. Colored and hardened panels have also been rigidized, by deforming them in a diamond pattern without cracking the film or reducing its color intensity.

It has also been found that treatment of the coated steel by the process of the invention renders it less retentive of finger marks caused by greasy deposits from the skin. It is presently believed that the cathodic treatment renders the coating less porous and thus less absorbent to these greasy deposits and the process may also usefully be applied to stainless steel bearing other porous coatings that are not themselves attacked and removed by the acid solution.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

l. A process for improving the corrosion resistance of stainless steel and the abrasion resistance of an immersion coating thereon which comprises forming a coating on the steel surface by immersion in chromic acid and then electrolytically treating the coated stainless steel as a cathode in a chromic acid bath for 21 period of time not of sufficient duration to produce a visible chromium deposit on the surface, but of sufficient duration to harden the immersion coating.

2. A process according to claim 1 wherein said electrolytic treatment comprises immersing the coated stainless steel as a cathode in a bath containing from about 25 to about 750 g/l. of chromic acid and 0.1 to 100 g/l. of sulfuric acid, maintaining the bath temperature from about 10 to about 100C., and passing current through the bath at a current density of from about 0.6 to about 10 A/dm for a period of at least one-half minute.

3. The process of claim 2 wherein the concentration of chromic acid is at least l00 g/l., the concentration of sulfuric acid is about 1 to 10 g/L, the bath temperature is in the range from about 20 to about 80C., the current density is from about 1 to about 5 A/dm and the time of treatment is from about 2 to about 30 minutes.

4. The process of claim 3 wherein the chromic acid concentration in the bath is about 250 g/l.

5. The process of claim 4 wherein the bath temperature is about 40C., the sulfuric acid concentration is about 2.5 g/l., the current density is in the range from about 2.4 to about 4.8 A/dm and the time of treatment is from 4 to 15 minutes.

6. The process of claim 1 wherein the immersion coating is produced by immersion in an aqueous solution containing about 200 to about 400 g/l. chromic acid, about 350 to about 700 g/l. sulfuric acid, at atemperature of about 65 to about 80C. for a time of at least about 5 minutes.

7. A process according to claim 6 wherein said immersion treatment is conducted for a time sufficient to produce at least a blue color on said stainless steel.

8. A process according to claim 7 wherein said aqueous solution contains about 300 g/l. chromic acid, about 550 g/l sulfuric acid and is used at a temperature of about C.

9. A process according to claim 1 wherein the immersion coating is produced in an aqueous solution containing 50 to 1 ,000 g/l. chromic acid, up to 600 g/l. sulfuric acid and up to 50 g/l. manganous sulfate.

10. A stainless steel product having an abrasionresistant film thereon hardened by the process of claim 1. 

2. A process according to claim 1 wherein said electrolytic treatment comprises immersing the coated stainless steel as a cathode in a bath containing from about 25 to about 750 g/l. of chromic acid and 0.1 to 100 g/l. of sulfuric acid, maintaining the bath temperature from about 10* To about 100*C., and passing current through the bath at a current density of from about 0.6 to about 10 A/dm2 for a period of at least one-half minute.
 3. The process of claim 2 wherein the concentration of chromic acid is at least 100 g/l., the concentration of sulfuric acid is about 1 to 10 g/l., the bath temperature is in the range from about 20* to about 80*C., the current density is from about 1 to about 5 A/dm2 and the time of treatment is from about 2 to about 30 minutes.
 4. The process of claim 3 wherein the chromic acid concentration in the bath is about 250 g/l.
 5. The process of claim 4 wherein the bath temperature is about 40*C., the sulfuric acid concentration is about 2.5 g/l., the current density is in the range from about 2.4 to about 4.8 A/dm2, and the time of treatment is from 4 to 15 minutes.
 6. The process of claim 1 wherein the immersion coating is produced by immersion in an aqueous solution containing about 200 to about 400 g/l. chromic acid, about 350 to about 700 g/l. sulfuric acid, at a temperature of about 65* to about 80*C. for a time of at least about 5 minutes.
 7. A process according to claim 6 wherein said immersion treatment is conducted for a time sufficient to produce at least a blue color on said stainless steel.
 8. A process according to claim 7 wherein said aqueous solution contains about 300 g/l. chromic acid, about 550 g/l sulfuric acid and is used at a temperature of about 70*C.
 9. A process according to claim 1 wherein the immersion coating is produced in an aqueous solution containing 50 to 1,000 g/l. chromic acid, up to 600 g/l. sulfuric acid and up to 50 g/l. manganous sulfate.
 10. A stainless steel product having an abrasion-resistant film thereon hardened by the process of claim
 1. 